Draghi's Optimism Is Unlikely to Prevent Yields From Falling Further

The euro-zone bond market is back in uncharted territory as worries about the euro-zone economy build. Two-year German yields are flirting with turning negative; 10-year yields at 1.07% are lower even than during the sovereign-debt crisis. Nothing Mario Draghi said Thursday seems likely to prevent them from falling further.

The president of the European Central Bank is still looking for euro-zone growth and inflation to pick up. But Mr. Draghi's optimism, based around economic survey data that has been consistently...